Extend channel definition with:

o max antenna gain
o driver private opaque data

Note this grows the size of a channel to 16 bytes; which makes the
default channel table 4Kbytes (up from 3Kbytes).

sys/net80211/_ieee80211.h

@@ -135,6 +135,9 @@ struct ieee80211_channel {
 	int8_t		ic_minpower;	/* minimum tx power in .5 dBm */
 	uint8_t		ic_state;	/* dynamic state */
 	uint8_t		ic_extieee;	/* HT40 extension channel number */
+	int8_t		ic_maxantgain;	/* maximum antenna gain in .5 dBm */
+	uint8_t		ic_pad;
+	uint16_t	ic_devdata;	/* opaque device/driver data */
 };
 
 #define	IEEE80211_CHAN_MAX	256